Many disk systems store metadata at the end of each disk. Old metadata should be erased before reusing the disk for a mirror. Most problems are caused by two particular types of leftover metadata: GPT partition tables and old metadata from a previous mirror.

GPT metadata can be erased with gpart(8) . This example erases both primary and backup GPT partition tables from disk ada8 :

A disk can be removed from an active mirror and the metadata erased in one step using gmirror(8) . Here, the example disk ada8 is removed from the active mirror gm4 :

If the mirror is not running, but old mirror metadata is still on the disk, use gmirror clear to remove it:

In this example, FreeBSD has already been installed on a single disk, ada0 . Two new disks, ada1 and ada2 , have been connected to the system. A new mirror will be created on these two disks and used to replace the old single disk.

The geom_mirror.ko kernel module must either be built into the kernel or loaded at boot- or run-time. Manually load the kernel module now:

Create the mirror with the two new drives:

gm0 is a user-chosen device name assigned to the new mirror. After the mirror has been started, this device name appears in /dev/mirror/ .

MBR and bsdlabel partition tables can now be created on the mirror with gpart(8) . This example uses a traditional file system layout, with partitions for / , swap, /var , /tmp , and /usr . A single / and a swap partition will also work.

Partitions on the mirror do not have to be the same size as those on the existing disk, but they must be large enough to hold all the data already present on ada0 .

Make the mirror bootable by installing bootcode in the MBR and bsdlabel and setting the active slice:

Format the file systems on the new mirror, enabling soft-updates.

File systems from the original ada0 disk can now be copied onto the mirror with dump(8) and restore(8) .

Edit /mnt/etc/fstab to point to the new mirror file systems:

If the geom_mirror.ko kernel module has not been built into the kernel, /mnt/boot/loader.conf is edited to load the module at boot:

Reboot the system to test the new mirror and verify that all data has been copied. The BIOS will see the mirror as two individual drives rather than a mirror. Because the drives are identical, it does not matter which is selected to boot.

See Troubleshooting if there are problems booting. Powering down and disconnecting the original ada0 disk will allow it to be kept as an offline backup.

In use, the mirror will behave just like the original single drive.

In this example, FreeBSD has already been installed on a single disk, ada0 . A new disk, ada1 , has been connected to the system. A one-disk mirror will be created on the new disk, the existing system copied onto it, and then the old disk will be inserted into the mirror. This slightly complex procedure is required because gmirror needs to put a 512-byte block of metadata at the end of each disk, and the existing ada0 has usually had all of its space already allocated.

Load the geom_mirror.ko kernel module:

Check the media size of the original disk with diskinfo:

Create a mirror on the new disk. To make certain that the mirror capacity is not any larger than the original ada0 drive, gnop(8) is used to create a fake drive of the exact same size. This drive does not store any data, but is used only to limit the size of the mirror. When gmirror(8) creates the mirror, it will restrict the capacity to the size of gzero.nop , even if the new ada1 drive has more space. Note that the 1000204821504 in the second line is equal to ada0 's media size as shown by diskinfo above.

Since gzero.nop does not store any data, the mirror does not see it as connected. The mirror is told to “forget” unconnected components, removing references to gzero.nop . The result is a mirror device containing only a single disk, ada1 .

After creating gm0 , view the partition table on ada0 . This output is from a 1 TB drive. If there is some unallocated space at the end of the drive, the contents may be copied directly from ada0 to the new mirror.

However, if the output shows that all of the space on the disk is allocated, as in the following listing, there is no space available for the 512-byte mirror metadata at the end of the disk.

In this case, the partition table must be edited to reduce the capacity by one sector on mirror/gm0 . The procedure will be explained later.

In either case, partition tables on the primary disk should be first copied using gpart backup and gpart restore.

These commands create two files, table.ada0 and table.ada0s1 . This example is from a 1 TB drive:

If no free space is shown at the end of the disk, the size of both the slice and the last partition must be reduced by one sector. Edit the two files, reducing the size of both the slice and last partition by one. These are the last numbers in each listing.

If at least one sector was unallocated at the end of the disk, these two files can be used without modification.

Now restore the partition table into mirror/gm0 :

Check the partition table with gpart show. This example has gm0s1a for / , gm0s1d for /var , gm0s1e for /usr , gm0s1f for /data1 , and gm0s1g for /data2 .

Both the slice and the last partition must have at least one free block at the end of the disk.

Create file systems on these new partitions. The number of partitions will vary to match the original disk, ada0 .

Make the mirror bootable by installing bootcode in the MBR and bsdlabel and setting the active slice:

Adjust /etc/fstab to use the new partitions on the mirror. Back up this file first by copying it to /etc/fstab.orig .

Edit /etc/fstab , replacing /dev/ada0 with mirror/gm0 .

If the geom_mirror.ko kernel module has not been built into the kernel, edit /boot/loader.conf to load it at boot:

File systems from the original disk can now be copied onto the mirror with dump(8) and restore(8) . Each file system dumped with dump -L will create a snapshot first, which can take some time.

Restart the system, booting from ada1 . If everything is working, the system will boot from mirror/gm0 , which now contains the same data as ada0 had previously. See Troubleshooting if there are problems booting.

At this point, the mirror still consists of only the single ada1 disk.

After booting from mirror/gm0 successfully, the final step is inserting ada0 into the mirror.

Synchronization between the two disks will start immediately. Use gmirror status to view the progress.

After a while, synchronization will finish.

mirror/gm0 now consists of the two disks ada0 and ada1 , and the contents are automatically synchronized with each other. In use, mirror/gm0 will behave just like the original single drive.

If the system no longer boots, BIOS settings may have to be changed to boot from one of the new mirrored drives. Either mirror drive can be used for booting, as they contain identical data.

If the boot stops with this message, something is wrong with the mirror device:

Forgetting to load the geom_mirror.ko module in /boot/loader.conf can cause this problem. To fix it, boot from a FreeBSD installation media and choose Shell at the first prompt. Then load the mirror module and mount the mirror device:

Edit /mnt/boot/loader.conf , adding a line to load the mirror module:

Save the file and reboot.

Other problems that cause error 19 require more effort to fix. Although the system should boot from ada0 , another prompt to select a shell will appear if /etc/fstab is incorrect. Enter ufs:/dev/ada0s1a at the boot loader prompt and press Enter . Undo the edits in /etc/fstab then mount the file systems from the original disk (ada0 ) instead of the mirror. Reboot the system and try the procedure again.

The benefit of disk mirroring is that an individual disk can fail without causing the mirror to lose any data. In the above example, if ada0 fails, the mirror will continue to work, providing data from the remaining working drive, ada1 .

To replace the failed drive, shut down the system and physically replace the failed drive with a new drive of equal or greater capacity. Manufacturers use somewhat arbitrary values when rating drives in gigabytes, and the only way to really be sure is to compare the total count of sectors shown by diskinfo -v. A drive with larger capacity than the mirror will work, although the extra space on the new drive will not be used.

After the computer is powered back up, the mirror will be running in a “degraded” mode with only one drive. The mirror is told to forget drives that are not currently connected:

Any old metadata should be cleared from the replacement disk using the instructions in Metadata Issues. Then the replacement disk, ada4 for this example, is inserted into the mirror:

Resynchronization begins when the new drive is inserted into the mirror. This process of copying mirror data to a new drive can take a while. Performance of the mirror will be greatly reduced during the copy, so inserting new drives is best done when there is low demand on the computer.

Progress can be monitored with gmirror status, which shows drives that are being synchronized and the percentage of completion. During resynchronization, the status will be DEGRADED, changing to COMPLETE when the process is finished.

Permanent labels can be a generic or a file system label. Permanent file system labels can be created with tunefs(8) or newfs(8) . These types of labels are created in a sub-directory of /dev , and will be named according to the file system type. For example, UFS2 file system labels will be created in /dev/ufs . Generic permanent labels can be created with glabel label. These are not file system specific and will be created in /dev/label .

Temporary labels are destroyed at the next reboot. These labels are created in /dev/label and are suited to experimentation. A temporary label can be created using glabel create.

To create a permanent label for a UFS2 file system without destroying any data, issue the following command:

A label should now exist in /dev/ufs which may be added to /etc/fstab :

Now the file system may be mounted:

From this point on, so long as the geom_label.ko kernel module is loaded at boot with /boot/loader.conf or the GEOM_LABEL kernel option is present, the device node may change without any ill effect on the system.

File systems may also be created with a default label by using the -L flag with newfs. Refer to newfs(8) for more information.

The following command can be used to destroy the label:

The following example shows how to label the partitions of a boot disk.

The glabel(8) class supports a label type for UFS file systems, based on the unique file system id, ufsid. These labels may be found in /dev/ufsid and are created automatically during system startup. It is possible to use ufsid labels to mount partitions using /etc/fstab . Use glabel status to receive a list of file systems and their corresponding ufsid labels:

In the above example, ad4s1d represents /var , while ad4s1f represents /usr . Using the ufsid values shown, these partitions may now be mounted with the following entries in /etc/fstab :

Any partitions with ufsid labels can be mounted in this way, eliminating the need to manually create permanent labels, while still enjoying the benefits of device name independent mounting.